CHAP, i.] DIGESTION. 239 



maxillary saliva being in most cases more active than the parotid. 

 In the rabbit, while the submaxillary saliva has scarcely any 

 action, that of the parotid is energetic. The saliva of the cat is 

 much less active than the above, and that of the dog still less ; 

 indeed the parotid saliva of the dog is wholly inert. In the horse, 

 sheep, and ox, the amylolytic powers of either mixed saliva, or of 

 any one of the constituent juices, are extremely feeble. 



Where the saliva of any gland is active, an aqueous infusion of 

 the same gland is also active. The importance and bearing of this 

 statement will be seen later on. From the aqueous infusion of 

 the gland, as from saliva itself, the ferment may be approximately 

 isolated. In some cases at least some ferment may be extracted 

 from the gland even when the secretion is itself inactive. 



The readiest method indeed of preparing a highly amylolytic 

 liquid tolerably free from proteid and other impurities, is to mince 

 finely a gland known to have an active secretion, such for instance 

 as that of a rat, dehydrate it by allowing it to stand under absolute 

 alcohol for some days, and then, having poured off most of the 

 alcohol, and removed the remainder by evaporation at a low tempe- 

 rature, to cover the pieces of gland with strong glycerine. A 

 mere drop of such a glycerine extract rapidly converts starch into 

 sugar. 



Gastric Juice. 



There is no difficulty in obtaining what may fairly be con- 

 sidered as a normal saliva ; but there are many obstacles in the 

 way of determining the normal characters of the secretion of the 

 stomach. When no food is taken the stomach is at rest and no 

 secretion takes place. When food is taken, the characters of the 

 gastric juice secreted are obscured by the food with which it is 

 mingled. The gastric membrane may it is true be artificially 

 stimulated, by touch for instance, and a secretion obtained. This 

 we may speak of as gastric juice, but it may be doubted whether 

 it ought to be considered as normal gastric juice. And indeed as 

 we shall see even the juice, which is poured into the stomach 

 during a meal, varies as digestion is going on. Hence the charac- 

 ters which we shall give of gastric juice must be considered as 

 having a general value only. 



Gastric juice, obtained by artificial stimulation from the healthy 

 stomach of a fasting dog, by means of a gastric fistula, is a thin 

 almost colourless fluid with a sour taste and odour. 



In the operation for gastric fistula, an incision is made through the 

 abdominal walls, along the linea alba, the stomach is opened, and the 

 lips of the gastric wound securely sewn to those of the incision in the 

 abdominal walls. Union soon takes place, so that a permanent opening 



